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More Controversy Over Earth's "Adaptive Infrared Iris"
Chou, M.-D., Lindzen, R.S. and Hou, A.Y.  2002.  Reply to: "Tropical cirrus and water vapor: an effective Earth infrared iris feedback?"  Atmospheric Chemistry and Physics 2: 99-101.

Fu, Q., Baker, M. and Hartmann, D.L.  2002.  Tropical cirrus and water vapor: an effective Earth infrared iris feedback?  Atmospheric Chemistry and Physics 2: 31-37.

Lindzen et al. (2001) used upper-level cloudiness and sea-surface temperature (SST) data from the eastern part of the western Pacific to develop a negative feedback concept which suggests that "the cloudy-moist region appears to act as an infrared adaptive iris that opens up and closes down the regions free of upper-level clouds, which more effectively permit infrared cooling, in such a manner as to resist changes in tropical surface temperature."  However, Hartmann and Michelsen (2002), in an analysis of spatial patterns of anomalous cloudiness and winds associated with the negative correlation between cloud-weighted SST and high-cloud fraction, claim that the correlation noted by Lindzen et al. results from variations in subtropical clouds that are not physically connected to deep convection near the equator, and that it is thus "unreasonable to interpret these changes as evidence that deep tropical convective anvils contract in response to SST increases."

What was done
Fu et al. (2002) continue to chip away at the adaptive infrared iris concept of Lindzen et al., arguing that "the contribution of tropical high clouds to the feedback process would be small since the radiative forcing over the tropical high cloud region is near zero and not strongly positive."  They also claim to show that "the water vapor and low cloud effects are overestimated [by Lindzen et al.] by at least 60% and 33%, respectively."  As a result, they obtain a feedback factor "in the range of -0.15 to -0.51, compared to [Lindzen et al.'s] larger negative feedback factor of -0.45 to -1.03."

Chou et al. (2002) reply that Fu et al.'s approach of specifying longwave emission and cloud albedos "appears to be inappropriate for studying the iris effect."  Also, they say that from the point of view that "thin cirrus are widespread in the tropics and that low boundary clouds are optically thick, the cloud albedo calculated by [Fu et al.] is too large for cirrus clouds and too small for boundary layer clouds," so that "the near-zero contrast in cloud albedos derived by [Fu et al.] has the effect of underestimating the iris effect."  They ultimately agree that Lindzen et al. "may indeed have overestimated the iris effect somewhat, though hardly by as much as that suggested by [Fu et al.]."

What was learned
The debate over the reality and/or magnitude of the adaptive infrared iris effect proposed by Lindzen et al. as a powerful means for thwarting CO2-induced increases in the atmosphere's greenhouse effect continues apace.  There has been some convergence in the two extreme views; but the controversy appears likely to continue for yet some time.

What it means
When some of the meteorological community's best minds continue to clash over the nature of a potentially important negative feedback phenomenon that could have far-reaching consequences for the CO2-climate debate, politicians clamoring for actions to reduce anthropogenic CO2 emissions should perhaps cool their heels until the relevant science becomes more settled.

Hartmann, D.L. and Michelsen, M.L.  2002.  No evidence for IRIS.  Bulletin of the American Meteorological Society 83: 249-254.

Lindzen, R.S., Chou, M.-D. and Hou, A.Y.  2001.  Does the earth have an adaptive infrared iris?  Bulletin of the American Meteorological Society 82: 417-432.

Reviewed 30 October 2002